A pale gold-tinted plating coating has been conventionally formed on the outermost layer of an abrasion-resistant layer for improving the scratch resistance of exterior parts, personal ornaments such as glasses, accessories, and watches, decorative trims, sporting goods, and the like in order to make an appearance with a high quality feel. The pale gold-tinted plating coating has been often formed by a wet plating method using a gold alloy of gold, and nickel, copper, silver, cobalt, cadmium, tin, or the like, as coating composition.
However, the alloy plating coating obtained by the wet plating method has had a color tone changed depending on the thickness of a coating, a current value, the shape of a base to be plated, and the like, it has not been easy to control the color tone, and therefore, unevenness in the color tone has been prone to occur in production.
Further, the gold alloy plating coating obtained by the wet plating method has had a low hardness, a Vickers hardness of 250 Hv or less in formation with a thickness of 10 μm, and has been poor in abrasion resistance, and therefore, the coating having a thickness of 30 μm or more has been generally formed and has been economically disadvantageous since gold which is a noble metal has been expensive.
Thus, Patent Literature 1 proposes that a golden coating having composition consisting of 80 to 95% by weight of gold, 5 to 20% by weight of nickel, and the other unavoidable components is formed on a titanium nitride (TiN) film by a dry plating method, whereby a pale gold-tinted decorative coating that is excellent in abrasion resistance and has a high quality feel is obtained.
The gold-tinted coating of Patent Literature 1 has had advantages that control of a color tone has been easy compared to conventional wet plating and abrasion resistance has been also improved; however, since a gold alloy has been used, the cost thereof has been still high, and it has not been able to be considered that the abrasion resistance has been sufficient.
Further, as a method for forming a coating presenting a gold color using a metal other than gold, a method of coating a nitride such as titanium nitride (TiN) or tantalum nitride (TaN) has been conventionally utilized. The method has had a defect in a production process that the partial pressure of a nitrogen gas has been preferably precisely controlled since the rate of nitrogen included in the nitride has greatly influenced a color tone.
Thus, Patent Literature 2 proposes that a first metal including at least one element of Group IVa elements of the periodical table and a second metal including at least one element of Group Va elements or Group VIa elements are used as targets, the two metals are simultaneously used as the targets in nitrogen atmosphere, reactive sputtering of an object is carried out simultaneously with the two metals in the nitrogen atmosphere, and the two metals are allowed to act as a buffer on a reaction system in which the Group IVa element reacts with nitrogen, whereby a coating is formed under relaxed conditions for controlling the partial pressure of nitrogen.
However, the method of Patent Literature 2 has had a problem that since the coating has been formed using the target including an element of Group IVa elements of the periodical table and the target of a metal including Group Va elements or Group VIa elements, the two metals in the obtained coating have prevented a uniform alloy nitride from being formed, it has been difficult to control a color tone, productivity has been poor, and it has been impossible to stably form a pale golden coating with a high quality feel.